The invention relates to a laser power control unit and a method thereof, and more particularly, to a laser power control unit and a method thereof for implementing a high-speed sample/hold circuit and a digital control mechanism to perform automatic power control.
With the rapid development of technology, the need for electronic information is constantly increasing. Most importantly, electronic information should be quickly stored in a very short time so as to preserve the information for later transmission. Therefore, the storage of electronic information becomes important, and the most popular way to store electronic information is to use an optical recording device.
Generally speaking, an optical recording device often makes use of a laser diode (LD) as the source of light, but the output power of LDs depends on the working temperature, which means that its output power varies slightly over different working temperatures. If the LD output power varies too severely, there will be some errors when writing the data, which results in errors when reading the data. Therefore feedback control circuits are adopted in order to keep the appropriate output power. As is well known in the art, this kind of feedback control circuit is known as an automatic power control (APC) device.
In order to record information on an optical recording medium, the output laser of the LD is modified to have different output power values and recording pulses. Moreover, the power and the widths of the recording pulses need to be control accurately. One key requirement to accomplish automatic power control is to measure the output power of the LD accurately.
Please refer to FIG. 1. FIG. 1 is a diagram of an automatic power control device according to the related art. In FIG. 1, the automatic power control device 100 is electrically coupled to an optical pick-up unit (OPU) 102, which has an LD for projecting a laser on an optical record medium (not shown) so as to perform reading and writing. The automatic power control device 100 includes a sampling circuit 104, an analog-to-digital converter (ADC) 105, a control circuit 108, and a digital-to-analog converter (DAC) 110. Generally speaking, the laser light projected by the LD is received by the front photodiode (FPD) set on the OPU, and the FPD outputs a front photodiode output signal (FPDO). Afterwards, a sampling circuit 104 samples the FPDO, and the ADC 106 transforms the sampled FPDO into a digital signal that can be processed by the control circuit 108. The control signal generated by the control circuit 108 is transformed into an analog signal by the DAC 110 so as to adjust the power of the LD of the OPU 102.
In the related art automatic power control device 100, the sampling circuit 104 is used for sampling the voltage of the FPDO in order to generate a sample signal for feedback control. As shown in FIG. 1, the sampling circuit 104 comprises a variable resister 112, an operational amplifier 113, and a switch 114. The variable resister 112 is capable of generating a first divided voltage L, and the operational amplifier 113 amplifies the first divided voltage L according to a gain so as to generate a second divided voltage H, which means that the second divided voltage H is larger than the first divided voltage L. In the related art automatic power control device 100, the switch 114 changes the voltage inputted into the ADC 106 to be the first divided voltage L or the second divided voltage H corresponding to the FPDO in accordance with whether the OPU 102 outputs high or low power. That is to say, the switch 114 performs switching according to the operation status. Please refer to FIG. 2. FIG. 2 is a diagram of laser power of the optical pick-up unit shown in FIG. 1 according to the related art. Using a CD-RW optical disk driver as an example, the OPU 102 provides a bias power Pb, an erase power Pe, and a write power Pw, wherein the write power Pw is larger than the erase power Pe, and the erase power Pe is larger than the bias power Pb. When the CD-RW optical disk drive performs an erasing operation, the OPU 102 outputs an erase power Pe to warm up a CD-RW disk, and when the CD-RW optical disk driver performs a writing operation, the OPU 102 first outputs a write power Pw to warm up a CD-RW disk, and then outputs a bias power Pb to read the CD-RW disk. For the switch 114, when the OPU 102 outputs the erase Pe or write power Pw, the switch 114 permits the second divided voltage L to be transmitted to ADC 106. On the other hand, when PU 102 outputs the bias power Pb, the switch 114 permits the first divided voltage H to be transmitted to the ADC 106.
However, the sampling structure mentioned above makes an assumption that the response speed of the FPD of the OPU 102 almost equals the impulse variation speed of the laser diode of the OPU 102. That is to say, the FPDO varies according to the impulse variation of each impulse. Therefore, if the data are written at a high speed, the sampling circuit 104 may not be able to perform the sampling operation correctly because of the switching speed of the sampling circuit 114 may not be fast enough. Afterwards, the related art sampling circuit 104 is not capable of sampling the FPDO so as to generate the voltage value. Additionally, in a high-speed or high-intensity optical recording device, the response speed of the FPD may be slower than the impulse variation speed, which might result in that the FPDO remains stable for a very short time during each impulse. Therefore, if the speed of the related art sampling circuit 104 is not fast enough, the related art sampling circuit 104 is not capable of sampling the appropriate voltage values correctly.
As the impulse becomes shorter and shorter, the response speed of the FPD of the OPU is probably slower than the variation speed of the impulse of the laser diode. If the FPDO does not correctly respond the output power of the laser diode, the real output power is not capable of being correctly measured using the FPDO so that the related art automatic power control device 100 is not capable of being applied to high-speed optical recording devices.